1. Field of the Invention
The present invention relates to a semiconductor device.
2. Background of the Related Art
Among semiconductor devices having a module structure which are used for general-purpose invertors, wind power generation, solar power generation and electric railroads, a semiconductor device having a module structure, where a front surface electrode of a semiconductor element and a circuit pattern are electrically connected (bonded) by wire bonding using aluminum (Al) wire or the like, is known. The front surface electrode of the semiconductor device having a module structure is an Al electrode or a copper (Cu) electrode, for example, and a circuit pattern is constituted by such a conductor as Cu.
Another semiconductor device having a module structure that is proposed has a configuration, where a nickel (Ni) plating film, a stacked film of Ni plating film and gold (Au) plating film, or a staked film of Ni plating film, a palladium (Pd) plating film and Au plating film is formed on a surface of an Al electrode, and an Al wire is wire-bonded to this plating film, or a lead frame is soldered to this plating film instead of Al wire bonding. The structure of the conventional semiconductor device having a module structure will be described.
FIG. 14 is a cross-sectional view depicting a structure of the conventional semiconductor device having a module structure. FIG. 15 is a cross-sectional view schematically depicting the structure of the semiconductor element in FIG. 14. As illustrated in FIG. 14 and FIG. 15, the conventional semiconductor device having a module structure has a semiconductor chip (semiconductor element) 101, an insulated substrate 102, a Cu base 106 and an Al wire 107. In the insulated substrate 102, a circuit pattern 104 constituted by Cu is disposed on the front surface side of an insulating layer 103, and a rear copper foil 105 is disposed on the rear surface side thereof.
The semiconductor chip 101 has a front surface electrode 112 constituted by Al or Cu on the front surface of a semiconductor substrate 111, and has a rear surface electrode 113 on the rear surface. The rear surface electrode 113 of the semiconductor chip 101 is bonded with the circuit pattern 104 via a solder bonding layer 101a. The front surface electrode 112 of the semiconductor chip 101 is electrically connected to the circuit pattern 104 by wire bonding using an Al wire 107 or the like.
An Al wire 107 is bonded using ultrasonic vibration, and good bonding is implemented without generating a connection failure by optimizing such conditions as heat, ultrasonic vibration and pressure with respect to the wire diameter of the Al wire 107. The wire diameter of the conventional Al wire 107 is about 300 μm to 400 μm. The bonding conditions between the front surface electrode 112 and the Al wire 107 change depending on the type of metal used for the front surface electrode 112 and the thickness of the front surface electrode 112. The front surface of the Cu base 106 is bonded with the rear copper foil 105 via a solder bonding layer (not illustrated).
As such a semiconductor device having a module structure, a device, of which surface glossiness of Ni plating film, which is formed on an electrode that is a target of performing Al wire-bonding and is constituted by a brass plate or Cu plate, is 1.6 or more, or the surface roughness of Ni plating film is 0.2 μm or less, and the thickness of Ni plating film is 2 μm or more, is proposed (e.g., see Japanese Patent Application Laid-open No. 2004-87772 (Patent Document 1)). Patent Document 1 discloses that the surface glossiness, the surface roughness and the plate thickness, among the characteristics of the Ni plating film, have a major influence on the strength of the Al wire bonding.
Another proposed device has a metal protecting film deposited on the surface of an Al electrode film, and an Al wire, which is electrically bonded with an Al electrode film via the metal protecting film by thermocompression bonding or ultrasonic vibration, where the metal protecting film is an Ni plating film having a 5 μm or less thickness deposited by an electroless plating method, and the thickness of the Ni plating film is not thicker than the thickness of the Al electrode film (e.g., see Japanese Patent Application Laid-open No. 2009-76703 (Patent Document 2)). Patent Document 2 discloses that solder bonding between the Al electrode film and the Al wire is enabled by the Ni plating film formed on the surface of the Al electrode film.
However as application areas expand, demands for heavier current conduction, higher temperature operation and improved reliability are increasing, and the following problem is generated. For example, the heavier current conduction can be handled by using a thicker Al wire (e.g. about 500 μm diameter), or by using a Cu wire of which conductivity is higher than that of the Al wire, but strength of the wire increases causing stress on the semiconductor chip when the wire is bonded to the surface of the front surface substrate, resulting in a deterioration of the semiconductor chip.
This problem is solved if the strength of the electrode portion is increased by forming an Ni film on the surface of the front surface electrode. However bonding of an Ni film and a wire involves the bonding of dissimilar metals, hence if conventional wire bonding conditions to bond similar metals (Al electrode and Al wire, Cu electrode and Cu wire) are used, a bonding failure (unbonded portion) is easily generated at the bonded interface between the Ni film and the wire, and the Ni film and the wire are not bonded perfectly.
A bonding failure is easily generated at the bonded interface of the Ni film and the wire because the material characteristics are different between Ni, which is the material of the Ni film, and Al or Cu, which is the material of the wire. Ni is harder and less extendable than Al and Cu, therefore stress applied on the Ni film is not easily dispersed, and the Ni film easily cracks during wire bonding. This means that the diameter of the wire, the thickness of the Ni film, and the bonding conditions between the Ni film and the wire are critical, but a detailed examination result of these aspects has not yet been disclosed.
Furthermore, in the case of conventional Al wire, a crystal structure in an area around the bonded interface with the front surface electrode becomes finer by the wire bonding, compared with the state before the wire bonding, whereby strength in an area near the bonded interface of the Al wire increases. However the crystal structure in an area near the bonded interface of the Al wire becomes larger and softer, depending on the thermal history in the manufacturing steps after the wire bonding and on the high temperature (e.g. 175° C.) operation due to the electric heating of the semiconductor elements. This makes it easier to generate cracking inside the Al wire.
The inventors conducted a power cycle test on the semiconductor device where an Al wire is bonded to the surface of the front surface electrode, and discovered that cracks are generated more inside the Al wire as a number of repeated cycles increases in the power cycle test, where finally the Al wire breaks down and separates, causing an element breakdown. Therefore development of a highly reliable semiconductor device, implementing heavy current conduction and high temperature operation is demanded by improving the tolerance to thermal load due to repeat heating and radiation (power cycle) during operation (hereafter called “power cycle tolerance (life duration)”).
With the foregoing in view, it is an object of the present invention to provide a highly reliable semiconductor device which implements heavy current conduction and high temperature operation.